string path = "你的文件地址/測試文件.txt";
                var filestream = new FileStream(path, FileMode.OpenOrCreate);
                HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.OK);
                response.Content = new StreamContent(filestream);
                response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
                response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")  { FileName = "測試文件.txt" }; 
              return response;

string contentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
  byte[] fileContents="文件轉化后的byte";
  return File(outfile, $"{fileContents};Charset=utf-8", $"行政費用申請{DateTime.Now.ToString("yyyy-MM-dd")}.xlsx");